In recent years, numerous metal detector circuits have been developed to provide a selective or discriminatory response to metal objects in proximity to the search head. As a result of the greater resistive component of the signals produced by low Q objects, such as aluminum pull tabs and the small resistive component of the signals produced by high Q objects, such as coins, a circuit which detects the proper component of the received signal will produce a response of opposite polarity for these objects.
In addition, numerous circuits have been developed for the purpose of excluding the effects of ferromagnetic minerals, such as magnetite, resulting in a ground neutralizing detector system. In these systems, the circuit is adjusted so that the component of the received signal caused by ferromagnetic minerals is not detected, resulting in a ground neutralized response.
The design problems inherent to both selective and ground neutralizing detector circuit are similar. Of primary concern is the ability of the circuit to provide a consistent response despite changes in the quiescent input signal within the detector circuit. These changes may be caused by temperature changes, aging of components, or large quantities of ferromagnetic minerals in proximity to the search head. In addition, the circuit should not require critical adjustment or component selection to enable ease of construction and to insure reliability.
It is therefore an object of the present invention to provide a selective or ground neutralizing detector circuit which will operate reliably, which may be constructed economically and which will not require critical adjustment.
It is another object of the present invention to provide a metal detector circuit incorporating a switching circuit to enable the detector to neutralize the effects of ferromagnetic minerals.
It is still another object of the present invention to provide a metal detector having a switching circuit which may be utilized to provide selectively for the identification of detected objects in accordance with the Q of such objects.
It is still another object of the present invention to provide a metal detector system utilizing a clamping circuit, controlled by a ground neutralizing detector circuit, for the purpose of removing the effects of ferromagnetic minerals from the response of a selective detector circuit.
These and other advantages of the present invention will become apparent to those skilled in the art as the description thereof proceeds.